1. Field of the Invention
The present general inventive concept relates to an image forming apparatus and, more particularly, to an apparatus and method of controlling emission of a laser beam in a laser image forming apparatus.
2. Description of the Related Art
In general, a laser beam scanning device of a laser image forming apparatus forms an electrostatic latent image on a photosensitive drum by turning on/off a semiconductor laser so that the latent image is transferred to a recording medium to print a desired image.
A semiconductor laser control device, which should perform on/off switching operations of the semiconductor laser at a high speed in the laser beam scanning device of the laser image forming apparatus, includes a set of additional parts such as a driver IC, a resistor and a condenser, or a discrete circuit using a transistor.
A conventional apparatus for controlling emission of a laser beam in an image forming apparatus will be described in conjunction with FIG. 1.
FIG. 1 is a view of a conventional laser emission control apparatus for controlling emission of a laser beam in an image forming apparatus.
As shown in FIG. 1, the laser emission control apparatus includes a comparator 20, an optical power controller 30, a switch 40, a variable resistor 50, a reference voltage supply 60, and a laser emitter 70. In this process, the comparator 20 may be a NAND gate, and the optical power controller 30 may be an auto power controller (APC).
The switch 40 may be a transistor. The laser emitter 70 includes a laser diode LD for emitting a laser beam, and a photodiode PD for detecting intensity (voltage) of the laser beam emitted from the laser diode.
A specific operation of the laser emission control apparatus will be described.
First, the comparator 20 of the laser emission control apparatus shown in FIG. 1 compares a video reference voltage (video data control signal-A having a wave form B) with a control voltage (G).A value of the control voltage (G) is determined by a feedback voltage Vf that is proportional to the photodiode current E. The output of the comparator 20 is supplied to the optical power controller 30.
In this process, the control voltage (G) supplied to the comparator 20 is a difference between a reference potential VCC and the feedback voltage value Vf corresponding to a current E flowing through the photodiode PD (D) of the laser emitter 70. The feedback voltage Vf is essentially the potential difference across the variable resistor 50 and a resistor R1, when a current flows from the photodiode PD detecting that the laser is lit. The reference potential VCC provided from the reference voltage supply 60, is connected to the variable resistor 50 via the resistor R1. In this process, the resistor R1 is connected in series to the variable resistor 50 for preventing an excessive current due to a malfunction of the variable resistor 50. If the variable resistor 50 becomes almost zero, the excessive current may reach and damage the laser diode LD (C). The presence of the resistor R1 prevents the excessive current from being supplied in the case of a malfunction of the variable resistor 50.
When the laser diode LD (C) of the laser emitter 70 is turned “ON” by the optical power controller 30 of the semiconductor laser, a laser beam is emitted from the laser diode LD (C) and is detected by an optical detecting device, i.e., the photodiode PD (D).
Intensity of the laser beam detected by the photodiode PD (D) is determined by a potential regulated by the optical power controller 30. The comparator 20 compares the video reference voltage (video control signal A) with the control voltage (G). The result of the comparison is input to the optical power controller 30. In this process, the comparator 20 may be comprised of a NAND gate as described above.
According to the input value supplied by the comparator 20 within a range of high and low voltages, the optical power controller 30 provides an up counter value or a down counter value to a base terminal of a transistor of the switch 40. The output of the optical power controller 30 turns on the transistor so that the transistor allows a current to flow to the laser diode LD (C) of the laser emitter 70 to emit a laser beam.
That is, the current supplied to the laser diode LD (C) in the laser emitter 70 depends on the current value supplied to the base terminal of the transistor of the switch 40 from the optical power controller 30.
The output from the comparator 20 is supplied to the optical power controller 30 that, via the transistor 40, provides then an input current to the laser diode LD (C) of the laser emitter 70. The laser beam emitted by the laser diode due to the input current can be detected by the photodiode PD (D).
The control voltage (G) is controlled by a current E of the photodiode PD. The current E may be a feedback current. As shown in a loop of a broken line G, the control voltage (G) is feedback as an input to the comparator 20. The control voltage (G) is a voltage difference between a reference potential VCC provided from the reference voltage supply 60 and the voltage value corresponding to the current E from the photodiode PD (D) of the laser emitter 70 flowing across the variable resistor 50 and the resistor R1.
As shown in FIG. 1, the laser diode LD (C) of the laser emitter 70 is on/off controlled according to the input of the video control signal A. The latent image concentration should be reduced in comparison with an output image in a normal state when a laser printer is used in a toner save mode. Therefore, in the toner save mode, the video control signal A is transmitted from the optical power controller 30 to the transistor of the switch 40, for a time shorter than the on time for forming dots in a normal mode, of the laser diode LD (C).
In the case of the toner save mode, since the on time of the laser diode LD (C) is controlled for a time shorter than at least ¼ of that of the normal state, as shown in FIG. 1, the switching using the transistor 40 happens to a very high frequency and may malfunction. Therefore, concentration differences may occur due to differences between parts or transistors when the high speed frequency is input, and furthermore, there may be problems such as an empty white image, i.e., an image cannot be printed.